Carbon nanotubes

ABSTRACT

Carbon nanotubes can be self-aligned by making composites of carbon nanotube powders with particles and organic and/or inorganic carriers such as water or other solvents. After the mixture is applied onto a substrate by whatever ways, such as brushing, screen-printing, ink-jet printing, spraying, dispersing, spin-coating, dipping, and the like and combinations, a fragmentation process occurs when the composite material is dried or cured by certain ways to eliminate some or all of the carrier material. This results in microcracks forming between the fragments. CNT fibers that are bonded or set in the fragments on either side of a crack are aligned in the crack area, either by stretching the fibers or by allowing the fibers to spool out from one or both fragments.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to the following:

Provisional Patent Application Ser. No. 60/502,464, entitled“SELF-ALIGNMENT OF CARBON NANOTUBES,” filed on Sep. 12, 2003.

TECHNICAL FIELD

The present invention relates in general to carbon nanotubes, and inparticular, to a process for manufacturing a carbon nanotubecomposition.

BACKGROUND INFORMATION

Carbon nanotubes (CNTs) are being investigated by a number of companiesand researchers because of their unique physical, chemical, electrical,and mechanical properties (P. M. Ajayan, O. Z. Zhou, “Applications ofcarbon nanotubes,” Top Appl. Phys. 80, 391-425(2001)). Aligned carbonnanotubes have been demonstrated to play an excellent role in logicalcircuits, high performance structural and functional composites,electronic devices, etc. For example, CNTs can be used as cold electronsources for many applications such as displays, microwave sources, x-raytubes, etc. because of their excellent field emission properties andchemical inertness (Zvi Yaniv, “The status of the carbon electronemitting films for display and microelectronic applications,” TheInternational Display Manufacturing Conference, Jan. 29-31, 2002, Seoul,Korea). Aligned carbon nanotubes with excellent field emissionproperties can be fabricated using chemical vapor deposition (CVD)techniques on catalytically-activated substrate surfaces with processtemperatures over 500° C. (Z. F. Ren, Z. P. Huang, J. W. Xu et al.,“Synthesis of large arrays of well-aligned carbon nanotube on glass,”Science 282, 1105-1107(1998)). CNTs can also be aligned by a tapingprocess (so called “activation”) after screen-printing a CNT paste ontoa substrate (Yu-Yang Chang, Jyh-Rong Sheu, Cheng-Chung Lee, “Method ofimproving field emission efficiency for fabricating carbon nanotubefield emitters,” U.S. Pat. No. 6,436,221). Other methods have also beenattempted to align CNTs, but they have some of the followingdisadvantages:

-   -   1. It is difficult to achieve high uniformity illumination        required for display applications using a CVD process to grow        CNTs over large areas.    -   2. CVD growth of CNTs requires a high process temperature (over        500° C.), limiting the use of low-cost substrates such as        sodalime glass.    -   3. The organic residue on the substrate after activation        processes may give off residual gases in the sealed glass        display envelope during field emission operation. Furthermore,        it is difficult to uniformly activate the substrate over a large        area. For example, many display applications may require 40″ to        100″ diagonal plates.

In summary, using CNT materials that require CVD growth processesdirectly on the substrate material or that require activation of the CNTmaterial over a large area have disadvantages that can be overcome withthe materials and processes of the present invention disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a digital image of a microcrack and aligned CNT fibers;

FIG. 2 shows another digital image of a microcrack and aligned CNTs;

FIG. 3 shows another digital image of a microcrack and aligned CNTs;

FIG. 4 shows broken CNTs between two fragments;

FIG. 5 shows aligned CNTs;

FIG. 6 illustrates a schematic diagram of a CNT-Resbond coating beforeand after a shrinking process;

FIG. 7 illustrates field emission I-V curves of samples of the presentinvention;

FIG. 8 shows an optical image of CNT dots;

FIG. 9 illustrates a process for dispensing CNT composites in accordancewith an embodiment of the present invention;

FIG. 10 illustrates an I-V curve of a sample made in accordance withembodiments of the present invention;

FIG. 11 shows a field emission image of the sample of FIG. 10;

FIGS. 12A-12C illustrate a process in accordance with embodiments of thepresent invention;

FIG. 13 illustrates an I-V curve of a composite made in accordance withembodiments of the present invention;

FIG. 14 shows a field emission image of a sample made in accordance withembodiments of the present invention;

FIG. 15 illustrates an I-V curve of a composite made in accordance withembodiments of the present invention; and

FIG. 16 shows a field emission image of a sample made in accordance withembodiments of the present invention.

DETAILED DESCRIPTION

In accordance with the present invention, carbon nanotubes can beself-aligned by making composites of carbon nanotube powders withparticles and organic and/or inorganic carriers such as water or othersolvents. After the mixture is applied onto a substrate by whateverways, such as brushing, screen-printing, ink-jet printing, spraying,dispersing, spin-coating, dipping, and the like and combinations, afragmentation process occurs when the composite material is dried orcured by certain ways to eliminate some or all of the carrier material.This results in microcracks forming between the fragments. CNT fibersthat are bonded or set in the fragments on either side of a crack arealigned in the crack area, either by stretching the fibers or byallowing the fibers to spool out from one or both fragments. The CNTsalign and are parallel to each other and to the substrate. Some CNTs mayalso be perpendicularly aligned on the face of the fragments. In somecases where the crack is large, some of the CNT fibers are also brokenin the crack area resulting in dangling fibers that emanate from bothfragments on either side of the crack. It may also be the case that somefibers are pulled out from one of the two fragments on either side ofthe crack. This process has several advantages:

-   -   1. Very easy and low cost process to align CNTs.    -   2. The CNTs can be aligned on a very large area.    -   3. No activation processes are required on the CNT composite        material after the curing step to achieve good field emission        properties for cold cathode applications.

The section below describes one embodiment that may be used to makealigned CNTs.

1. Source of Materials

Single-wall carbon nanotubes (SWNTs) were obtained from CarboLex, Inc.,Lexington, Ky., U.S.A. These SWNTs were in the range of from about 1 nmto 2 nm in diameter and in the range of from about 5 μm to 20 μm inlength. Single-wall, double-wall, or multi-wall carbon nanotubes (MWNTs)from other vendors and prepared by other methods, and with otherdiameters and lengths, can also be used with similar results.

The other components of the composite prepared were contained in aninorganic adhesive material obtained from Cotronics Corp., Brooklyn,N.Y., U.S.A. having a name/identifier of Resbond 989 (“Resbond”) that isa mixture of Al₂O₃ particles, water, and inorganic adhesives. Compositesthat contain other particles may also be used, such as SiO₂. Theseparticles may be insulating, conducting or semiconducting. The particlesizes are less than 50 μm in size. Sizes may be much smaller than this.The carrier in the Resbond is water, but other carrier materials may beused and they may also be organic or inorganic. Other materials thatpromote other properties of this material, such as binders (e.g., alkalisilicates or phosphate) may also be present in the composite in smallquantities.

2. Preparation of the Mixture of Carbon Nanotubes with the Resbond andDeposition onto Substrate

1) Grinding of the Mixture

A 1 gram quantity of CNT powders (40 wt. %) and a 1.5 gram quantity ofResbond (60 wt. %) were put together into a mortar. The mixture wasground using a pestle for half an hour in order that the mixture lookslike a gel, meaning that the CNTs and Al₂O₃ particles did not separatewith each other. Please note that a different weight ratio of CNTs toResbond may also work. Additionally, water or other carrier materialsmay also be added into the mixture to dilute it in order to adjust theviscosity. The mixture was then ready for depositing onto the substrate.

2) Applying the Mixture onto the Substrate and Curing

A brush was used to paint the mixture onto a conventional Si substrate(10-100 kΩ cm) with an area of 2×2 cm². Other substrate materials suchas ceramics, glass, metals, alloys, polymers, or other semiconductorsmay also be used. Other ways to put the mixture onto the substrate suchas screen-printing, spraying, spin-coating, ink-jet printing, dipping,and dispensing may also be utilized or performed. The thickness of thecoating was about 20 μm to 40 μm. The substrate was dried at roomtemperature in the air, but it may also be dried/cured in an oven atincreased temperature (approximately 100° C. or higher) in order to morequickly eliminate the water. If the solvent contains organic(s), theneven higher temperatures may be set to remove it. For example, up to300° C. will be set to remove epoxy. The oven or curing vessel maycontain a vacuum pump to exhaust the air out of the oven and form avacuum inside the oven during the drying/curing process. The oven orcuring vessel may also provide a gas environment or flow around thesample that further promotes curing or drying. This gas environment orflow may or may not be partially or completely from inert gases such asthe noble gases or nitrogen. Ultraviolet or infrared light may also beused to aid the curing process.

3. Microstructure of the CNT-Resbond Coating

Scanning electron microscopy (SEM) was used to analyze the surfacemorphology of the sample. A JEOL made, JSM6320F & JSM-35 model SEM wasused for the experiment. Because Al₂O₃ is an insulating material, a 20nm-thick Au thin film was evaporated on the top of the coating beforetesting. FIGS. 1 through 5 show SEM images of the sample created using aprocess of the present invention.

FIG. 1 shows an SEM image of a microcrack within the sample and alignedCNT fibers between two fragments. One can see fibers that are alignedand attached to both fragments and some fibers that are dangling fromone of the fragments, often in the same picture.

FIG. 2 shows an SEM image of another microcrack within the sample andaligned CNTs between two fragments.

FIG. 3 shows an SEM image of further microcracks in the sample andaligned CNTs among the three fragments. No dangling CNT fibers are seenin this image.

FIG. 4 shows broken CNTs between two fragments in the sample. This imageshows mostly dangling or broken fibers with possibly one fiber that isstretched across the gap between the fragments.

FIG. 5 shows perpendicularly aligned CNTs against the substrate.

FIGS. 1 and 2 show SEM images of aligned CNTs between the nearest twofragments. Most of the CNTs are parallel to each other. They are alsoparallel to the substrate. The microcracks occurred during thefragmentation of the CNT-Resbond mixture, during the drying/curing step.FIG. 3 shows aligned CNTs among the three nearest fragments with threedifferent aligned directions. If the fragmentation process was moredramatic, the CNTs could be extracted from one fragment and remain onthe other fragment, or broken with the ends of the fiber remaining inthe fragment on both sides of the crack (see FIG. 4). There are severalaligned CNTs contacting both sides of the fragments. Other CNTs areeither extracted or broken and left on one or both sides of thefragments. Overall, those CNTs are still aligned. FIG. 5 shows theperpendicularly aligned CNTs against the substrate. Thoseextracted/broken CNTs are particularly useful for field emissionapplications because one end of the CNT fiber is exposed to the air orvacuum. Furthermore, as shown in the photographs, the density of thedangling fibers is not so great that they shield the applied electricfields needed for field emission applications from each other. TheCNT-Resbond coating may be separated to many islands after the shrinkingprocess. This is also good for field emission properties of theCNT-Resbond coating because it can minimize the screening effect duringthe field emission of the CNTs and expose more CNT fibers outside theResbond matrix without further needed activation processes.

FIG. 6 illustrates a magnified schematic diagram of a CNT-Resbondcoating before (a) and after (b) a shrinking process. The film aftershrinking shows the nanotubes in the cracks between the islands. Somenanotubes may extend across the crack and others may extend onlypartially into the crack.

4. Field Emission Test of the CNT-Resbond

1) To find the best CNT content in the mixture, the following differentweight ratios of the CNTs to Resbond were designed to find whichconcentration was the best for field emission:

-   -   10 wt. % CNTs+90 wt. % Resbond    -   25 wt. % CNTs+75 wt. % Resbond    -   40 wt. % CNTs+60 wt. % Resbond    -   75 wt. % CNTs+25 wt. % Resbond

The above mixtures were prepared as described above and were brushedonto ITO/glass substrates with an area of 1 cm×1 cm and were dried inthe air for 10 minutes. The thickness of the coatings were in the rangeof from about 20 microns to about 30 microns. After drying, the sampleswere ready for field emission testing. To compare field emissionproperties, a CNT sample without any Resbond content was also made usingthe same brush process as the other samples.

All the samples were tested by mounting them with a phosphor screen in adiode configuration with a gap of about 0.63 mm between the anode andcathode. The test assembly was placed in a vacuum chamber and pumped to10⁻⁷ Torr. The electrical properties of the cathode were then measuredby applying a negative, pulsed voltage (AC, 2% duty factor) to thecathode and holding the anode at ground potential and measuring thecurrent at the anode from field emitted electrons from the cathode. A DCpotential could also be used for the testing, but this may damage thephosphor screen. A graph of the emission current vs. electric field forthe samples is shown in FIG. 7. It can be seen that the sample with 40wt. % CNTs had the lowest extraction field for a given current, which isdesirable for field emission properties. All the samples that containedResbond had better field emission properties than CNTs without Resbond.

2) Demonstration of Applying the CNT Composite by Dispensing Process

The mixture of 40 wt. % CNT and 60 wt % Resbond was utilized anddispensed onto a substrate and the field emission was tested. Dispensingis an excellent process to deposit very small dots onto the substrateover large areas. The definition of such size dots is suitable formaking CNT high resolution field emission displays. A Musashi-madedispenser (model: SHOT mini™) was employed to deposit the mixture ontoconductive ITO (Indium tin oxide). Other dispenser machines can be used,including ink-jet approaches. Patterns are made by moving the dispensinghead and/or the substrate relative to each other and dispensing dots orlines of material at pre-defined locations.

The diameter of the nozzle was 300 μm. Six rows of the mixture weredispersed with 51 dots on each of them. It can be seen below in FIG. 8that the CNT dots on the substrate were larger (around 750 μm to about800 μm), but smaller dots can be achieved by adjusting the viscosity ofthe mixture and using smaller nozzles. FIG. 9 shows the dispensingprocess. Methods of making the nozzle and dispensing apparatus are wellknown to those who practice the art and not discussed further here.Multiple-nozzle heads may also be used to improve speed and throughputof the machine.

Field emission of the sample (having 51×6 dots on it) was tested. TheI-V curve is shown in FIG. 10. An electric field as low as 3 V/μm wasachieved at an emission current of 40 mA. FIG. 11 shows a field emissionimage of the sample, where a phosphor covered anode is positioned inproximity to the sample cathode, and an electric field is applied toinduce field emission. FIG. 11 shows a digital image of the emittedlight.

It can be seen that there is no edge emission and that emission sitedensity is excellent. Also note that no other activation process wasutilized to improve the field emission properties.

3) Field Emission from Screen-Printed CNT-Resbond Mixture

Screen-printing is a well-known technology that has been applied invarious fields. This method was used to print the mixture onto aselective area of the substrate through a mask. FIGS. 12A-12C show aschematic diagram of the substrate coated with CNT-Resbond mixture. The3′×3′ glass plate was screen-printed with 10 μm-thick Ag feedlines andthen it was coated with a 50 μm-thick black insulating layer onselective areas so it contained in total 64 pixels. Every pixel had 3sub-pixels. The size of every sub-pixel was 1×6.6 mm².

FIGS. 12A-12C show a schematic diagram of the steps used to screen-printthe device described above. The test results of this type device areshown below in FIGS. 13, 14, and 15.

In FIG. 12A, Ag feedlines 1201 are screen-printed onto the glasssubstrate 1202. In FIG. 12B, screen-printing of a 50 μm-thick insulatingovercoat 1203 is performed onto the Ag feedline-printed glass substrate1202. In the next step illustrated in FIG. 12C, CNTs 1204 arescreen-printed onto the Ag feedline opening.

A sample was prepared using CNT-Resbond deposited by a screen-printingprocess. A glass substrate was used with a 35 micron-thick blackinsulating overcoat (glass frit glaze) layer printed on printed silverfeedlines and the CNT material was printed onto the pixels using astencil mask (stainless steel sheet, with no mesh in the openings). TheCNT coating was about 50 microns to 70 microns. After printing anddrying/curing, the sample was then tested.

FIG. 13 illustrates an I-V curve of the sample with CNT-Resbonddeposited by printing process. FIG. 14 shows a digital image of a fieldemission image of a set of pixels using the CNT-Resbond composite.

The CNT-Resbond composite was also printed with very small pixels(300×1700 μm²) using a stencil mask. In the screen-printing process, aTiW thin film-coated glass was employed as the substrate. Printing wason top of the TiW thin film that was thick enough to be highlyconductive. The stencil mask was a stainless steel sheet with Tefloncoating on the surface. There were 26×24 pixels on the substrate. Fieldemission of the sample was tested. The I-V curve is shown in FIG. 15 anda field emission image is shown in FIG. 16. There is no edge emissionobserved in the image in FIG. 16. Emission site density is reasonable.

FIG. 6 illustrates that during the drying/curing process, the compositematerial may shrink and crack as a result of the shrinking. The fibersalong the crack are stretched or spooled out of the fragments on eitherside of the crack. This aligns them in the crack. In some cases thefibers are broken as a result of the crack formation or they are pulledout of one fragment on one or both sides of the crack.

-   (1) CNT-Resbond were coated onto the substrate-   (2) Fragmentation process begins when the coating is curing or    drying-   (3) Cracks were widened and CNTs were aligned-   (4) CNTs were broken or extracted from one fragment if the    fragmentation process is dramatic.

In summary, CNT composites are made from CNT fibers, other particles,carrier materials and possibly other materials for binding. After thecomposite is dispensed or printed or placed onto the substrate, thecomposite is dried or cured (carrier material is taken out), resultingin fragmentation of the composite on the substrate and a crack(s) isformed between the fragments. The CNT fibers in the cracks are stretchedor spooled from the fragments to align them in the crack region. Somefibers are broken or pulled out of the fragments to create danglingfibers that may be ideal field emitter structures in many cases.

1. A composition comprising carbon nanotubes and an inorganic adhesivematerial.
 2. The composition as recited in claim 1, wherein some of thecarbon nanotubes are exposed within microcracks formed in thecomposition.
 3. The composition as recited in claim 2, wherein at leastone of the exposed carbon nanotubes bridges across the microcrack. 4.The composition as recited in claim 2, wherein at least one carbonnanotube exposed within the microcrack is broken in two.